we haven't cured the common cold, or the flu, is that the viruses constantly mutate. If you don't believe in evolution, you shouldn't catch any more colds.
On this subject, though--there is an absolutely enormous problem within medicine concerning the appropriate allocation of research funding. It is one of the reasons we need government funding, because private research will seek to cure common and expensive diseases. But government funding is also influenced by non-objective factors. For example, schizophrenia affects fully 1% of the U.S. population, and causes vast disability, comparable to heart disease, diabetes, and cancer. But compared to cancer research, schizophrenia research gets 1/30th the funding.
The same issue is obvious in medical pay. People who treat children or other vulnerable, politically weak populations--pediatricians, psychiatrists, inner-city internists--earn less money than the average physician. Starting pediatrician salaries are similar to those of nurses. Those who primarily treat the diseases of upper-class white people--e.g. cardiologists, dermatologists, heart surgeons etc.--can make far more money.
There is legislation that supports "orphan drug" research, encouraging companies to seek cures for illnesses that are too uncommon to produce easy profit.
Mariner
(I am a physician, and treat primarily poor and disabled people. I earn 1/5 what my brother, a heart surgeon, makes. He isn't any smarter, more competent, or harder working than me--it's just the system.)